Fixed #17258 -- Moved `threading.local` from `DatabaseWrapper` to the...

Fixed #17258 -- Moved `threading.local` from `DatabaseWrapper` to the `django.db.connections` dictionary. This allows connections to be explicitly shared between multiple threads and is particularly useful for enabling the sharing of in-memory SQLite connections. Many thanks to Anssi Kääriäinen for the excellent suggestions and feedback, and to Alex Gaynor for the reviews. Refs #2879.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17205 bcc190cf-cafb-0310-a4f2-bffc1f526a37